This invention relates to the separation of nitrogen from air by rectification, and is particularly concerned with improved procedure for the separation of nitrogen from air employing a non-adiabatic air fractionating system, in conjunction with a reversing heat exchanger for removal of water vapor and carbon dioxide, from the feed air.
In the prior art for production of oxygen and nitrogen from air, carbon dioxide and water vapor have been removed from the feed air by external means, such as molecular sieves, as exemplified by U.S. Pat. No. 3,594,983. However, molecular sieves used for this purpose are bulky, heavy and relatively expensive.
In U.S. Pat. No. 3,508,412 for production of nitrogen by air separation, compressed air is cooled in a regenerative cooler in countercurrent heat exchange relation with oxygen-rich vapor and nitrogen.
The most economical method for removing carbon dioxide and water vapor from the feed air is to deposit the CO.sub.2 and water vapor, in solid form on the surface of the regenerative heat exchanger, and, by reversing the flow passages between the incoming feed air and the low-pressure oxygen-rich waste stream, these contaminants are sublimated off the heat exchange surface into the vapor phase. However, such regenerative heat exchangers have generally been employed with a high feed air pressure, e.g. of the order of about 10 atmospheres.
It is an object of the present invention to provide a process and system to separate nitrogen from air by rectification while reducing power consumption as low as possible, by reducing the pressure of the feed air, preferably to about 3 atmospheres, or less.
Another object is to employ reversing heat exchangers for carrying out water vapor and carbon dioxide removal from the feed air at pressures at or below 3 atmospheres.
Another object is to carry out separation of nitrogen from air using reversing heat exchangers in conjunction with an air fractionation system, for removal of carbon dioxide and water vapor while maintaining an air feed pressure of not more than about 3 atmospheres.
Yet another object is to enable production of both liquid and gaseous nitrogen product, while still maintaining air purification employing the above process and system utilizing reversing heat exchanges.